The study is designated to determine the median dose and dosage range of fentanyl citrate administered as a continuous I.V. infusion, required to provide adequate analygesia in patients with cancer experiencing severe pain. As fentanyl is 50-70 times more potent than morphine its dosage is extremely low. We have set up a gas-chromatographic method for its analysis from plasma that uses a N-P detector. The sensitivity of the drug appears to be 0.5ng/ml (S/N = 3). Standard calibration curves with spiked samples have shown linearity between 0.5 - 15 ng/ml. The procedure involved a single extraction with hexane and can potentially be automated. We are in the process of accumulating the necessary data required for the procedure validation.